Is this ever used as another way to confirm what someone else sys, like it is in English? As in "I must be going crazy!" "That you are, my friend; that you are."
This sentence fragment was deleted from the database some time ago, but it has yet to disappear from the system.
It has no real meaning on its own.
The "that", in this case, is to signify that the phrase means "the fact of your being", like the previous that in this sentence.